1. Field of the Invention
The invention relates generally to bladed turbomachinery and more specifically to a vibration reduction mechanism for a high-speed radial impeller to reduce or eliminate fatigue failures resulting from various excitation sources.
2. Background Information
Radial impeller blades are subjected to alternating forces from various sources including, for example, surge forces, stall forces, harmonic resonance, the presence of inlet vanes and valves, turbulence within the system and the like. The response for each vibratory mode of the blades is a function of the strength of the excitation force, the material properties of the blade and the vibration damping characteristics of the blade.
Vibration of the blades is a frequent cause of blade failure due to the creation of bending stresses in the blades. When these bending stresses exceed the maximum allowable values of the blade material, a crack will often develop. As such a crack enlarges under continued stress, a point is reached where the entire blade fractures and a substantial portion of the blade is loose within the machine. Machines of this type often operate at very high rotary speeds wherein a broken blade part can create substantial damage to the machinery.
In addressing this problem, the prior art has lashed impeller blades together to damp the vibration of the blades. Prior art lashings have taken several forms. For example, all of the blades have been lashed together by a single ring of material successively secured to each blade. This type of lashing is not always feasible due partly to the enlargement of the blade diameter during rotation of the impeller which imposes bending or hoop stress on the lashing member and self-imposed stresses resulting in a failure in the lashing member.
U.S. Pat. No. 3,131,461 to Miller discloses a lashing construction for damping blade vibrations in axial flow turbomachinery wherein each blade is lashed to its adjacent blades by a pair of solid pins extending between and connected to the two adjacent blades. In axial flow turbomachinery, such as illustrated in the Miller patent, due to the cantilevered, tapered blade construction the vibration across the tip of the blade is substantially the same. The difficulties with the axial impeller lashing system disclosed in the Miller patent include the fact that the system does not consider the vibration amplitude of the relative blade connecting positions. The Miller patent lashing system locates the connecting positions of each blade to an adjacent blade at points where the vibration of each blade can be equal and in phase and thus not able to provide frictional damping limiting effectiveness of the system.
The object of the present invention is to overcome the aforementioned drawbacks of the prior art.